Voltage stabilized liquid dielectrics



United States Patent VOLTAGE STABILIZED LIQUID DIELECTRICS Lawrence J.Heidt, Arlington, Mass., assignor to Simplex Wire and Cable Company,Cambridge, Mass., a corporation of Massachusetts No Drawing. Filed July21, 1966, Ser. No. 566,766 U.S. Cl. 25263.7 13 Claims Int. Cl. H01b 3/20ABSTRACT OF THE DISCLOSURE A method of increasing the allowable voltagestress rating, resistance to aging and electric discharge of oilsincluding the naturally occurring or synthetic paraffinic, naphthenic ormoderately aromatic hydrocarbon oils is disclosed. This is accomplishedby adding to the oil an additive in the amount of from .1 percent up tothe limit of solubility of the additive in the oil. Suitable additivesinclude o-nitrotoluene, o-nitroanisole, Z-nitrodiphenylamine,o-nitrobiphenyl, 2,4-dinitrotoluene, diphenylamine, benzoguanamine,triphenyl formazan, anthranilonitrile and mixtures thereof. The liquiddielectric compositions can be used for electrical insulation incombination with plastic or paper films or alone in cables,transformers, potheads, capacitors and otherwise.

The present invention relates to dielectric compositions and inparticular to liquid dielectric compositions containing additives whichenhance their voltage stability.

Liquid dielectric compositions are frequently used for electricalinsulation in combination with plastic or paper films or alone incables, transformers, potheads, capacitors and otherwise. Principalamong these dielectric compositions are oils such as mineral oils of theparaffinic or naphthenic types, and synthetic hydrocarbon oils such aspolybutene and polyisobutene oils, but these compositions are subject tocertain limitations of dielectric strength and deterioration underelectric discharge. Improvement of these properties would enable the useof smaller quantitles of the compositions thereby reducing the overallsize of the electrical component in which they are used.

It is the object of this invention to provide an oil additive forincreasing the allowable stress rating and the resistance to aging andelectric discharge of the more common liquid dielectric materials.

It is a further object of this invention to provide voltage stabilizeddielectric compositions which permit reduction in the seize and cost ofelectrical components in which they are utilized.

The additives of the present invention which enhance the dielectricstrength of liquid compositions such as hydrocarbon oils are certainsubstituted aromatic hydrocarbon compounds characterized in part byhaving an electron acceptor group and an electron donor grouppotentially hydrogen bonded together by a reversibly transferableproton. Some of these additive are among those disclosed in Heidtcopending application Ser. No. 372,301 filed June 3, 1964, as voltagestabilizing additives for solid olefin polymers such as polyethylene.Dielectric breakdown in solid polyolefin dielectrics was not thought tobe related, and indeed may not be related to dielectric breakdown inliquid dielectrics. Thus in a solid polyolefin dielectric breakdownoccurs under high voltage stress by the movement of electrons throughthe dielectric which produces ionization at points where the dielectricis not homogeneous, for example, at the location of voids or solidparticles which are inevitably found in solid polyolefin dielectricsbecause of the need of mechanical shaping, for example, by extrusion.Breakdown occurs when such ionization is suflicient to weaken thedielectric such 'ice that an avalanche of electrons escapes through itat a point of ionization producing a treeing burn out in the dielectric.The additives of the above Heidt application are thought to capture highenergy electrons escaping through the solid dielectric and to dissipatethe energy of the electron. The initial cause of dielectric breakdown inliquids obviously differs, as the hydrocarbon oils employed for thispurpose are highly refined and do not require mechanical working toshape them. Accordingly, it was therefore not anticipated that thesubstituted aromatic hydrocarbons which function as voltage stabilizersin solid polyolefin dielectric might similarly serve to stabilize liquidhydrocarbon dielectrics.

The preferred voltage stabilizer additives are, of course, thosematerials which enhance voltage stability up to the very highest appliedvoltages. Materials which are both effective stabilizers and alsoreadily available in commerce are most desirable. In accordance with thepresent invention it is postulated that effective additives have incommon the following:

(1) An electron acceptor group, especially a strongly unsaturatedradical, e.g. one containing a 1r bond, such as -NO CO, CN, phenyl andpolycyclic aromatics.

(2) An electron donor group, especially one containing a transferableproton such as amino and lower, i.e. up to eight carbon atoms, alkylradicals, e.g. -NH and --CH (3) Potential hydrogen bonding between theacceptor and donor group by a transferable proton, such as when theacceptor and donor groups are ortho with respect to one another, e.g. ona benzene ring.

(4) Reversibility of the proton transfer between the acceptor and donorgroups, such as in the keto-enol isomerization.

(5) The structure and bonds between the acceptor and donor groups whichfavor transfer of charge and energy, such as a planar or near planarstructure of a conjugated system of alternating single and double bonds,e.g. an aromatic ring structure.

(6) Adequate size and complexity of the conjugated system to provide forelectron capture and subsequent energy dissipation without producingirreversible bond rupture.

(7) Adequate dispersibility of the additive in the liquid dielectric toprovide a sufficient number of distributed centers for the capture ofelectrons moving in the dielectric under an electric field.

Generally speaking the relative electron afiinities of the atomicresidues are in the order of Cl being greater than 0. O is in turngreater than N which in turn is greater than C. The aceptor groups haveelectron affinities in the order of NO being greater than CN which isgreater than C=O in turn greater than phenyl. The relative electrondonor strengths of groups such as -N(CH are greater than NH which isgreater than CH in turn greater than phenyl. The Hammett sigma and sigmaplus values are a rough measure of the relative electron acceptor anddonor properties of the atoms and groups. These sigma values have beendetermined and tabulated by many people, especially P. R. Wells,Chemical Reviews, pp. 171-219, April 1963; see the tables on page 181and 189 of this article.

Effective voltage stabilizers in accordance with the present inventioninclude such aromatic hydrocarbons as O- nitrotoluene; O-nitroanisole;Z-nitrodiphenylamine; O- nitrobiphenyl; 2,4-dinitrotolnenediphenylamine, benzoguanamine, triphenyl formazan anthranilo nitrile.The above compositions may be used alone or admixed with each other, orfor ease in handling they also may be admixed with hydrocarbon oilspreferably aromatic oils as the aromatic structure itself provides avoltage stabilizing effect. The admixture of such oils and additives toform voltage stabilized compounds of solid olefin polymers such aspolyethylene is disclosed in copending Hunt application Ser. No. 367,718filed May 15, 1964.

It is important to the realization of improved voltage stability thatthe additive of the present invention be used in soluble amounts sincecrystallization during use may adversely affect the performance of theoils whether used as a cable insulation impregnant, a capacitorinsulation impregnant or as a filler for transformers or potheads.Generally, at least 0.1 part by weight of additive per 100 parts byweight of oil is necessary to impart significant voltage stabiilty and,therefore, it is a requirement of the additives of this invention thatthey be soluble in the oils to which they are added at least to theextent of 0.1%. The amount of additives used may vary between 0.1 partby weight per 100 parts by weight of oil up to the limit of solubilityof the additive in the oil and is preferably between .25 part and 4.0parts by weight of additive per 100 parts by weight of oil. The effectof increasing the amount of additive is most pronounced at additiveconcentrations between 1.0 part and 2.0 parts by weight per 100 parts byweight of oil. At concentrations above 3.0 parts by weight of additiveper 100 parts by weight of oil the effect of increasing the amount ofadditive decreases noticeably so that only negligible improvement involtage stability occurs at concentrations above 4.0 or 4.5 parts byweight of additive per 100 parts by weight of oil. The solubilities ofthe additives vary according to the particular oil used, the morearomatic oils having greater solvent power than the paraffin oils, andthis variation should be oils, cable oils such as Sun XX, a polybutene'oil having a low viscosity and a low temperature viscosity coefficientwhich is useful with paper or plastic tape cables or as a pothead fillerand a G & W 235, a medium viscosity polybutene oil suitable for use inhigh voltage cables especial- 1y cables using plastic films instead ofpaper tapes. The voltage stabilizing effect is not limited to those oilsspecifically designed as dielectrics, but has been found to occur inother hydrocarbon oils. Primol 355, a pure paraffinic white oil used forpharmaceutical purposes exhibits enhanced voltage stabiilty when solubeamounts of voltage stabiilzers of this invention are incorporated in it.Thus this invention can lead to the use of particular hydrocarbon oilsas dielectrics which were not heretofore suitable for the purpose.

To demonstrate the voltage stabilizing characteristics of the additivesof this invention, 15 minute stepwise tests were run on samples eachhaving six layers of paper impregnated with the dielectric liquidcompound under test. In each instance a test was run on one six layersample impregnated with the oil only and other samples containing theoil plus at least 1% of an additive according to the present invention.The samples were subjected stepwise to increasingly higher voltagestress using crossed cylindrical electrodes having three layers of paperon each electrode. The electrode assembly was immersed in the compoundbeing tested. Results are reported as the average of 5 or more tests oneach in Table 1 below in which the composition of the various compoundstested is set forth in the column identified by the letters A-I,inclusive, in parts by weight of the additives in the same row at theleft hand column of the table.

TAB LE I Components, Parts by Weight Base 011 (Transil 10-0) L. J.

Heidt, 19 July 1966 Base Oil (Primol 355) Aromatic Oil (CD101)2,4-dinitrot oluene Diphenylamine 2-nitrodiphenylamine O-nitrobiphenylO-nitrotoluene- O-nitroanisole. Dielectric Breakdown, volts per mil perhr.

Compounds Tested D E F G B C H I In those compounds containing (JD-101,the other additives were first mixed with the CD-lOl and then themixture was added to the base oil. CD101 is a hydrocarbon oil having anaromatic content in excess of a specific gravity of 1.035, a pour pointof 20 F., viscosity SUS of at 100 F. and 36 at 210 F.

A second series of 30-minute step-rise tests were run on samples eachhaving six layers of paper impregnated with liquid dielectric compounds.The tests were conducted in accordance with the procedure describedabove with reference to Table I. The results are similarly reported inTable II as the average of five or more tests on each kind of compound,the composition of the compounds in Table II being identified in thecolumns headed by the letters K-U, inclusive, and the explanations ofTable I are equally applicable.

TABLE II Compounds, Parts by Weight Base Oil (Sun XX) Base Oil (G & W235) Aromatic Oil (CD-101) 2,4-d1nitrot0luene DiphenylamineO-nitrotoluene Compounds Tested L M N 0 P Q, R S T U DielectricBreakdown, volts per mil perhr 540 595 665 705 705 591 688 725 715 715700 In those compounds containing OD101, the other additives were firstmixed with the (DD-101 and then the mixture was added to the base 011.

The negligible benefits to be gained from increased additiveconcentrations may be observed by comparison of Example K with ExamplesN-O and P-Q-R in Table II. Thus, in Examples P and Q the additionof 1and 2 parts by weight of diphenylamine resulted in dielectric breakdownvalues increased by 10 and 27.5% respectively above the base oil valveof 540; whereas in Example R the relatively large addition of 5 parts byweight of diphenylamine resulted in a mere 7% further increase in thedielectric breakdown value.

The following are examples of typical voltage stabilizing oil additivesand oil additive combinations used accordng to the present invention:

EXAMPLE 1 Benezoguanamine.-Used alone or in combination with any one ofO-nitrotoluene, O-nitrobiphenyl, 2,4-dinitrotoluene (the latter only tothe extent that it is soluble) in amounts of .25 part of 4.5 parts byweight of total additives per 100 parts by weight of oil.

EXAMPLE 2 Triphenyl formazan.Used in amounts of .25 part to 0.5 part byweight per 100 parts by weight of oildepending upon the particular oilusedalone or in combination with .25 part to 4.0 or 4.25 parts by weightof any one of O-nitrotoluene, O-nitrobiphenyl, 2,4-dinitrotoluene (thelatter only to the extent that it is soluble).

parts by weight per 100 parts by weight of oil, alone or in combinationas defined by Examples 1 through 3.

EXAMPLE 5 O-nitrobiphenyl.-Used in amounts of .25 part to 4.5 parts byweight per 100 parts by weight of oil alone or in combination as definedin Examples 1 through 3 above.

EXAMPLE 6 2,4-dinitrotoluene.Used in amounts of .25 part to 2.0 parts byweight per 100 parts by weight of oil-in greater amounts wheresolublealone or in combination as defined in Examples 1 through 3.

O-nitroanisole.-Used in amounts of .25 part to 2.0 parts by weight per100 parts by weight of oil due to its limited solubility in most oils.

EXAMPLES 9-19 Examples of typical voltage stabilized transformer oils inaccordance with the present invention are shown in Table III below.

TABLE III Example Numbers Components, Parts byWeight 9 10 11 12 13 14 1516 l7 18 19 Kazan 2iiilc(g{laI(lgi1D1 O1BC1)). ::III:I: 10g 100 100 100100 100 100 100 100 100 100 2,4-dim'trotoluene Diphenylamine2-nitrodiphenylamine O-nitrobiphenyl.

O-nitrotoluene. O -nitroanisole Benzoguanimine Triphenyl formazan Inthose compounds containing CD-101, the other additives were first mixedwith the (JD-101 and then the mixture was added to the base oil.

EXAMPLE 3 Dipheny1amine.Used alone or in combination with any one ofO-nitrotoluene, O-nitrobiphenyl, 2,4-dinitro- EXAMPLES 20-29 Examples ofvoltage stabilized capacitor impregnant oils are shown in Table IVbelow.

O-nitrebiphenyl O-nitrotoluene- O-nitroanisole Benzoguanimine Triphenylformazan 25 Anthranilo nitrile *In those compounds containing (JD-101,the other additives were first mixed with the (JD-101 and then themixture was added to the base oil.

toluene (the latter only to the extent that it is soluble) in amount of.25 part to 4.5 parts by weight of total additive per 100 parts byweight of oil.

EXAMPLE 4 Examples 30-40 Examples of typical voltage stabilized oilssuitable for use in oil impregnanted cables having paper or plastic filmO-nitrotoluene.Used in amounts of .25 part to 4.5 insulation are shownin Table V below.

TABLE V Example Numbers Components, Parts by Weight 31 32 33 34 35 BaseOil (Sun XX) Aromatic Oil (CDl0l)* 2,4-dinitrotoluene Diphenylamine2-nitrodiphenylamine O-nitrobiphenyl- O-nitrotoluene. O-nitroanis0leBenzoguanimin Triphenyl formazan- Anthranilo nitrile-.

In those compounds containing (JD-101, the other additives were firstmixed with the (JD-101 and then the mixture was added to the base 011.

The voltage stabilized oils of Examples 30-40 are useful as impregnantsin cables which are insulated with synthetic tapes such as polyphenyleneoxide filaments. These tapes, when impregnated with the stabilized oilsof this invention are able to carry very high voltages with low powerlosses.

Examples 41-51 Examples of typical voltage stabilized pothead fillingoils are shown in Table VI below.

TABLE VI Example Numbers Components, Parts by Weight Base Oil (IndopolH150) 100 100 100 100 100 Base Oil (Aronite l2E) Aromatic Oil (CD-101).2, 4-dinitrotoluene Diphenylamine... 2-nitrodiphenylam O-nitrobiphenyl.O-nitrotoluene O-nitro anisole Benzoguanimine Triphenyl formazan...Anthranilo nitrile *In those compounds containing (JD-101, the otheradditives were first mixed with the OD-lOl and then the mixture wasadded to the base oil.

The above examples of voltage stabilized oils suitable for use intransformer, pothead capacitor and oil impregnanted cable insulationsare intended to be merely typical of the compositions which can beprepared according to the present invention. It will be obvious thatvariations of the particular oil and additive compositions and theconcentrations used can be substantially varied within the scope of thisinvention. In addition dielectric oils which are stabilized according tothe present invention can contain other conventional additives such asantioxidants and the like.

It will be noted that some of the additives consist of or includeamines, e.g. diphenylamine. In using these additives in contact withcopper components precaution should be taken to avoid dissolution of thecopper in the oil by reaction with the amine. The use of, for example,tinned copper is suggested. While the use of such precautionary measuresmay add to the expense of the additive composition, the presence ofamines is frequently helpful since they act as thermal stabilizers forthe oils.

I claim:

1. A method of increasing the allowable voltage stress rating,resistance to aging and electric discharge of an oil selected from thegroup consisting of naturally occurring and synthetic paraffinic,naphthenic and moderately aromatic hydrocarbon oils and mixtures thereofcomprising adding to said oil an amount between .1 percent up to thelimit of solubility of an additive selected from the group consisting ofo-nitrotoluene, o-nitroanisole, 2-nitrodiphenylamine, o-nitrobiphenyl,2,4-dinitro- 5. The method of claim 1 wherein the additive includeso-nitrotoluene.

6. The method of claim 1 wherein the additive includes o-nitroanisole.

7. The method of claim 1 wherein the additive includes2-nitrodiphenylamine.

8. The method of claim 1 wherein the additive includes o-nitrobiphenyl.

9. The method of claim 1 wherein the additive includes2,4-dinitrotoluene.

10. Te method of claim 1 wherein the additive includes diphenylamine.

11. The method of claim 1 wherein the additive includes benzoguanamine.

12. The method of claim 1 wherein the additive includes triphenylformazan.

13. The method of claim 1 wherein the additive includesanthranilonitrile.

References Cited Chemical Abstracts, vol. 46, col. 10744b, FifthDecennial Index, pp. 4250 and 1050.

LEON D. ROSDOL, Primary Examiner.

J. D. WELSH, Assistant Examiner.

US. Cl. X.R.

